A POSSIBLE GEOTECTONIC INTERPRETATION OF THE SOUTH APUSENI MOUNTAINS (ROMANIA) D. G. DORDEA and V. SPRANCEANA

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1 A POSSIBLE GEOTECTONIC INTERPRETATION OF THE SOUTH APUSENI MOUNTAINS (ROMANIA) D. G. DORDEA and V. SPRANCEANA Caransebes st., Bucharest, sector 1, Romania; Abstract: After a long debate concerning the nature (type) of the Mesozoic magmatites from the South Apuseni Mountains - the existence of exclusive island arc volcanism (IAV) products versus ophiolites (O) followed by IAV products, the second theory imposed. There still remained a lot of uncertainties regarding the spatial extension and relations between these products and the Mesozoic sedimentary deposits, complicated by the nappe structure of the area. A composed structure formed by narrow, prolonged slides of Laramian mélange suture, deciphered by a detailed Remote Sensing and geophysical interpretation covering all the area, is proposed. Key words: ophiolites, volcanic arc island rocks, Mesozoic sedimentary deposits, suture mélange, slides structure, South Apuseni Mountains. Introduction The South Apuseni Mountain (SAM), covering an area of about 3,300 km 2, are composed by J K 1 N-type MORB sequences of ultramafic-mafic isotropic cumulates of gabbros and diorites / sheeted dykes / pillow lava forming an ophiolitic complex (Saccani et al., 2001) and volcanic island arc rocks (Savu, 1995; Savu et al., 1988; Savu et al., 1991; Savu et al., 1992a; Savu et al., 1992b; Savu et al., 1992c; Cioflica G. et al., 2001; Saccani et al., 2001) of J 3 and K 1 age. The geologic setting of SAM is completed by a variety of J - K sedimentary deposits which compose, together with the Mesozoic magmatic suites, the so considered nappe structure. Different interpretations of this nappe structure have been advanced, the most comprehensive belonging to Sandulescu (1984). Balintoni and Berza (2001) consider this nappe system of Laramian age. The present structure of SAM is accomplished by Laramian magmatics (banatites), by Miocene sedimentary deposits and Sarmatian-Quaternary calc-alkaline / alkaline volcanics.

2 The arguments for a nappe structure are: stratigraphycal discordances mapped in different outcropping locations, different composition of ophiolites (namely the presence of spilitic rocks in certain nappe units - Sandulescu (1984), the presence of olistolithes and the prevalence of specific rocks like ophiolites and flysch-rocks (Sandulescu, 1984). A lot of unclear issues still persist on nappe postulate because: - There is no detailed mapping to discriminate O, IAV the Mesozoic sedimentary deposits (generally covered with soil and vegetation); - No detailed photogeological mapping was performed till now; - Widespread ophiolitic tectonic breccias occurring within Capalnas-Techereu Nappe has been considered cartographically as pyroclasics (Borcos, M. et al., 1981) - Breccias-type of the limestone olistolithes is entirely ignored; - The olistolith elongation of few kilometers, despite the narrow width, of hundreds of meters, is also ignored; - The obvious recurrence of some discordant stratigraphic successions (Fenes area - Dambau) are interpreted as successive faulting; - The presence of crystalline rocks in the Capalnas-Techereu unit (mainly ophiolitic) still remains exotic, unaccountable in a unitary remnant of ocean floor crust. The nappe units of SAM surround the North Apuseni Mountains (Internal Dacides) by their southern and eastern sides: Grosi unit, Cris unit, Bucium unit, Capalnas-Techereu unit, Curechiu- Stanija unit, Fenes unit, Trascau (Bedeleu) unit, Fundoaia unit, Boyes unit, Cabesti unit, Bejani unit. The photogeological interpretation reveals the development of some long, imbricate slides on the entire Transilvanides area (fig. 3). The slides are formed exclusively of Mesozoic rocks. Successive slides of ophiolites, limestone and flysch rocks, as well as successive ophiolitic and island arc volcanic rocks may develop. Only the thickness of the limestone slides may exceed m. Within the ophiolitic slides, the typical succession of an ophiolitic complex (peridotite/gabbro/sheeted dikes/pillow lava) cannot be

3 distinguished, though the specific ophiolitic types have been randomly described all over the Apusenide area. However, the basaltic pillow-lavas prevail, though frequently transformed into tectonic breccias. Both the carbonatic and the carbonatic - ophiolitic olistolithes develop linearly as continuous or discontinuous slides, being certainly the result of a tectonic process, not of the development of olistostrome or wildflysch. The slides design of the Mesozoic rocks is quite significant and it is slightly disturbed by the Banatitic volcanism (Laramian), the Neogen volcanism or by the Neogen sedimentary deposits. It subsequently develops along major E-W direction on the South segment of the Apusenides (the Capalnas Techereu area, the Ampoi basin) and along SW-NE direction in the eastern segment (Trascau Mts), following the major suture line of the continental blocks of the Internal Dacides/ Median Dacides. Geophysical data Both regional shaded relief aero-magnetic (fig. 1) and gravity (fig. 2) images succeed to reveal the whole outcropping area of the ophiolitic complex of the SAM. Moreover, the magnetic contrast and the gravity mass contrast between ophiolites and its lithological background, allow their regional outlining both to NE (beneath the Transilvanian basin) and to SW (beneath the Pannonian basin) to be defined. Within the alignments of regional maximum gravity, associated mainly to the ophiolitic piles, the subsequent detailed measurements (upon the Valisoara Luncoiul de Jos area) have revealed a succession of gravity maximum and minimum relative parallel to the main direction (WSW - ENE) of the ophiolites. The relative gravity minimum may be associated either to the presence of island arc volcanics, or to sedimentary rocks. This alternation is better reflected by the horizontal gravity gradient (fig. 4), which suggests a possible suture structure. Conclusion The entire area between the Internal Dacides and Median Dacides represents a suture mélange formed by slides of oceanic crust (equally composed by ophiolites and sedimentary rocks), which randomly solders to the continental margins at north and at south. The design of the majority of the nappes forehead, represented cartographically, may constitute a structural local reality, but the real

4 nature of the nappes, as a whole, represents in fact, a recurrence in an uncountable number of such phases. References Balintoni I., Berza T. (2001) Geodynamics and tectonics of the Apuseni Mountains (Romania), Rom.Jour. of Mineral Deposits, Vol.79. Suppl.2, 3-5 pp., Bucharest Cioflica G., Lupulescu M., Udubasa S., (2001) The Mesozoic magmatites complex and associated metalogenesis of the South Apuseni Mountains, Rom. Jour. Of Mineral deposits, Vol.79, Suppl.2, 6-13 pp., Bucharest Saccani E., Nicolae I., Tassinari R., (2001) - Tectono-magmatic setting of the Jurassic ophiolites from the South Apuseni Mountains (Romania): petrological and geochemical evidence, Ofioliti, 26 (1), 9-22 pp., Sandulescu M. (1984) Geotectonica Romaniei, Ed.Tehnica, 360 pp. Bucharest Savu H., Udrescu C., Lemne M., Romanescu O., Neacsu V. (1988) Petrology, geochemistry and tectonics of ophiolites and late Kimmerian island arc volcanics from the Glodinesti-Salistioara tectonic rise (Mures zone), D.S Sed. IGG, 5.Tect. si Geol.Reg., pp., Bucharest Savu H., Stoian M. (1991) REE and Hf contents in the ocean floor rocks (Liassic ophiolites) from the Trascau Mountains and their petrologic significance, Rev.Roum. Geol., Tome 35, pp., Bucharest Savu H., Udrescu C., Neacsu V., Stoian M. (1992a) The Costei basalts; products of a pre-mures ocean continental intra-plate volcanism, Rev.Roum.Geol.a, Tome 36, pp. Bucharest Savu H., Udrescu C., Neacsu V.(1992b) On the presence of ocean floor rocks (Liassic ophiolites) in the Trascau Mountains (Mures Zone); their petrology and geochemistry, Rom.J. Petrology, 75, pp. Bucharest Savu H., Udrescu C., Neacsu V. (1992c) Petrology and geochemistry of the late Kimmerian island arc volcanics from the Trascau Mountains (Mures Zone), Rom.J. Petrology, 75, pp. Bucharest Savu H. (1995) Genesis and structure of the Mures Zone, Rom.Jour.Of Mineral Deposits, pp., Bucharest Borcos M., Berbeleac I., Bordea I., Mantea Gh., Bostinescu S. (1981) Harta geologica Romania, sc.1:50.000, foaia Zlatna

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